Bi-directional, isolated DC-to-DC inverters are useful in a wide range of applications. Examples of such applications include, but are not limited to, uninterruptible power supplies, battery charging systems, and auxiliary power supplies for hybrid electrical vehicles.
It has been recognized that two-inductor boost converters suffer from limited output voltage regulation range when operated at low duty cycles. More particularly, when a load is below the minimum input power of a converter circuit of this type, further decreases in the load result in abnormal increases in output voltage due to excess energy storage in the inductors. Yan et al. have proposed a solution to avoid these increases in output voltage. (“Isolated Two-Inductor Boost Converter with One Magnetic Core,” Eighteenth Annual Applied Power Electronics Conference and Exposition, Feb. 9–13, 2003, Miami Beach, Fla., pp. 879–885.) An auxiliary transformer is utilized in series with two inductors to magnetically couple the two input current paths, ensuring that the current in the two inductors is the same. Thus, inductor current is eliminated when the load draws no current. A magnetic component disclosed by Yan et al. provides an isolated two-inductor boost converter with one transformer. This component has two inductor windings intrinsically coupled and is implemented with one gap in a three-leg magnetic core. However, circuits utilizing an auxiliary transformer and those utilizing the magnetic component of Yan et al. may require that these windings or components be capable of carrying high currents.
Another example of a DC-to-DC converter is disclosed by Li et al. (“A Natural ZVS High-power Bi-direction dc-to-dc Converter with Minimum Number of Devices,” presented at IEEE Industry Applications Society Annual Meeting, Sep. 30–Oct. 4, 2001, Chicago, Ill., pp. 1874–1881.) This converter is operated with dual half-bridges placed on each side of an isolation transformer. When power flows from the low-voltage side to the high-voltage side, the circuit operates in boost mode. Conversely, when power flows in the opposite direction, the circuit works in buck mode to recharge a battery used to provide power to the low-voltage section. To support bidirectionality, a complex thirteen-step commutation procedure is used that depends upon the magnitudes of various currents at specified times.